This invention relates to assemblies for supporting electrical circuit boards within tubes, and is concerned more particularly, but not exclusively, with assemblies for supporting circuit boards within measurement probes to be accommodated down-hole in a borehole such that the circuit boards are substantially resistant to mechanical vibration and shock.
In the surveying of boreholes various measurements are taken down-hole and are processed prior to being transmitted to the surface, for example by means of a mud-pulse signalling transmitter. The appropriate measuring instrumentation and processing circuitry is mounted within a drill collar incorporated in the drill string in the vicinity of the drill bit. More particularly the circuit boards accommodating the measurement and processing circuitry are suppored within an elongate tube which is in turn mounted within a sleeve by means of a pivotal coupling at one end to allow limited pivotal movement of the probe within the sleeve. The sleeve is then itself mounted within the drill collar with the interposition of a suitable damping medium between the outside of the sleeve and inside wall of the drill collar.
In operation of such instrumentation downhole, however, the circuit boards are subjected to very high levels of vibration and this can lead to the creation of strains at the junctions between the circuit components and the circuit boards which can eventually result in fatigue failures at these junctions. Such failures are an important cause of malfunction within measurement probes down-hole.
There are other applications in which electrical circuit boards mounted within tubes or otherwise are subjected to vibration in operation, and various measures have been proposed for protecting the circuitry against failure due to vibration. For example, U.S. Pat. Specification No. 4,072,378 proposes the injection into wiring cavities of a flexible foamed resin to protect the electronic components of apparatus for installation in a motor vehicle engie compartment. Furthermore U.S. Patent Specification No. 3,539,874 suggests mounting a cradle of an indicating instrument, for use in aircraft for example, by means of a locking arrangement and two resilient pads which engage the circular periphery of each circuit board. The resilient pads are capable of being compressed to enable the circuit boards to be engaged within the locking position. U.S. Patent Specification No. 2,832,012 discloses a magnetic amplifier structure in which a series of power stage reactors are mounted within a tube and are separated by means of layers of resilient rubber extending transversely of the tube and cushioning the reactors against mechanical vibration.
However, none of these prior arrangements would be suitable for protecting a measurement probe against device failure due to the mechanical vibration likely to be encountered down-hole.
It is an object of the invention to provide an assembly for supporting an electrical circuit within a tube which is suitable for use in such a measurement probe.